Okadaic acid, a protein phosphatase inhibitor, enhances transcription of a receptor gene containing sequence A of the human prolactin promoter

Human PRL (hPRL) gene expression is controlled by cAMP and Ca2+. This control is mediated by two cis-elements: a Pit-1 binding site (-62 to -35) and sequence A (-110 to -85), present in the hPRL promoter. We have investigated whether protein phosphatases could be involved in this regulation. GC-type rat pituitary tumor cells were transfected with sequence -138 to -35 of the hPRL gene promoter, upstream from a thymidine kinase promoter and a chloramphenicol acetyltransferase (CAT) reporter gene. Addition of okadaic acid (OA), a specific inhibitor of protein phosphatases 1 and 2A, stimulates transient expression of the CAT gene. The dose-response curve shows a maximal effect at 25 nM OA (2.2-fold stimulation above controls). The OA effect is also observed with a natural 4500-base pair hPRL promoter. A single copy of the hPRL promoter sequence -115 to -85 (sequence A) confers to a thymidine kinase-CAT construct an identical response to OA, whereas a single copy of the proximal Pit-1 binding site does not. Synergism is observed between cAMP and OA in activating PRL gene transcription. This synergism is also observed with a single copy of sequence A. The effect of cAMP is not mediated by an L-type Ca2+ channel, since addition of the Ca2+ channel antagonist verapamil does not decrease it, nor does complexing extracellular Ca2+ significantly reduce it. Furthermore, OA and the Ca2+ channel opener BAY K8644 exert additive effects.